We have recently described an acquired immunodeficiency syndrome in a colony of captive macaque monkeys. This naturally occurring disease provides an important opportunity to study acquired immunodeficiency states in an animal model. We have recently isolated a Type D retrovirus from affected macaques; preliminary data suggest that this agent may be etiologic in the disease. We will: 1) Characterize Type D retrovirus isolates from macaques with the immunodeficiency syndrome and compare them to previously described retrovirus isolates; 2) Detail the natural history of the disease caused by this agent; 3) Investigate mechanisms of oncogenic transformation in the naturally occurring and transmitted lymphomas in macaques; 4) Investigate the mechanisms by which this agent causes immune suppression; 5) Derive attenuated, vaccine strains of the virus.